1. Field of the Invention
The present invention relates to a control system for an automatic transmission for a vehicle, and more particularly, to a control system for an automatic transmission for a vehicle which is designed such that when the slipping of the driven wheels of the vehicle has been determined, the shifting of the automatic transmission is prohibited.
2. Description of the Related Art
If driven wheels are slipped when a vehicle provided with a vehicle speed detecting means for detecting a vehicle speed based on a rotational speed of a rotary member of the automatic transmission starts to travel on an icy surface having a smaller friction coefficient, the rotational speed of the rotary member is increased. Therefore, the vehicle speed detecting means detects a higher vehicle speed, but an actual vehicle speed remains lower, because of a loss in grip between the driven wheels and a road surface. In this case, because the vehicle speed detecting means detects the higher vehicle speed, there is a possibility that the automatic transmission is upshifted, and as a result the number of rotations of the driven wheels is further increased to exert a negative influence on the durability of a differential or the like.
There is a well-known control system disclosed in Japanese Patent Publication No. 64-7259 which is designed so that when the rotational acceleration of the rotary member of the automatic transmission becomes equal to or greater than a predetermined value, it is determined that the driven wheels are in their slipped states, and upshifting is prohibited based on this determination.
In the control system for the automatic transmission provided with such shift prohibiting means, if the canceling of the prohibition of the upshifting is not properly performed, there is a possibility that the prohibition of the upshifting is continued even after elimination of the slipping, or the prohibition of the upshifting is canceled before elimination of the slipping.